1. Field of the Invention
A disclosed embodiment relates to a radar device and a method of processing a signal.
2. Description of the Related Art
Recently, in a radar device detecting objects, the detection accuracy of data (hereinafter, referred to as “object data”) corresponding to an object within a scanning range has been improved in accordance with the improvement of the output of a transmission wave, which is accompanied with the improvement of the performance of an RF (Radio Frequency) circuit generating a transmission signal of the radar device, the improvement of the signal processing capability of a signal processing unit for a reception signal that is based on a reflected wave acquired by reflecting a transmission wave on an object, and the like. For example, in a case where such a radar device is mounted in a vehicle, object data detected by the signal processing unit of the radar device is output to a vehicle control device that is electrically connected to the radar device. Then, the vehicle control device controls the behavior of the vehicle based on information of a relative distance, a relative speed, an angle, and the like of the object with respect to the vehicle.
Here, an overview of the process of detecting object data is as follows. A transmission signal corresponding to a transmission wave of a radar device and a reception signal corresponding to a reception wave are mixed by a mixer, and an FFT (Fast Fourier Transform) process is performed for a bit signal that is a signal of a difference between the transmission signal and the reception signal, whereby a plurality of transformed signals are generated. Then, signals exceeding a predetermined threshold are derived as peak signals from among the plurality of transformed signals, and object data is detected by pairing the peak signals of an UP zone and a DOWN zone.
For example, in a case where a microscopic object having a relative small reflection area for a transmission wave such as snow or rain is present on the front side of the vehicle within the scanning range of the radar device, the following process is performed in a conventional radar device. In other words, while the signal level of a transformed signal becomes higher as a distance between the vehicle and the microscopic object decreases, the signal level of the transformed signal does not exceed a predetermined threshold and thus is not derived as a peak signal. As a result, object data corresponding to a microscopic object such as snow or rain is not detected. As a material describing the technology relating to the present application, there is Japanese Patent Application Laid-Open No. 62-15480.
However, there is a case where a microscopic object that is not detected as object data by the conventional radar device is detected as object data in accordance with the improvement of the detection accuracy of an object described above. In other words, as the signal level of a transformed signal corresponding to a microscopic object such as snow or rain that is present at an extremely short distance (for example, a distance of 0.6 m on the front side of the vehicle) from the vehicle exceeds a predetermined threshold in accordance with the improvement of the performance of the radar device, there is a case where the signal processing unit derives the transformed signal corresponding to the microscopic object as a peak signal and detects object data corresponding to the microscopic object. Then, as the object data of the microscopic object is output to the vehicle control device, the radar device outputs object data that is not originally needed to be output to the vehicle control device, and there is a case where the vehicle control device performs unnecessary control for the vehicle.
For example, in a case where the vehicle travels on the front side, a microscopic object such as snow or rain is a stationary object having a speed of about 0 km with respect to the front side of the vehicle and has a relative speed corresponding to the speed of the vehicle. For example, in a case where the vehicle travels at 60 km/h, the microscopic object is at 60 km/h with respect to the vehicle when viewed from the vehicle, and, when the forward direction (traveling direction) of the vehicle is set for a positive relative speed, the relative speed is −60 km/h. In a case where the object is present at an extremely short distance (for example, a distance of 0.6 m on the front side of the vehicle) from the vehicle, bit frequencies generated to be frequencies on both positive and negative sides as frequencies of differences between the transmission signal and the reception signal in the UP zone and the DOWN zone have the following correspondence relation with the microscopic object. In other words, a bit frequency of a frequency present on the negative side corresponds to a microscopic object in the UP zone, and a bit frequency of a frequency present on the positive side corresponds to a microscopic object in the DOWN zone. In other words, the bit frequency on the positive side of the UP zone and the bit frequency on the negative side of the DOWN zone are not bit frequencies that correspond to a microscopic object.
Before an FFT process in which a signal of a bit frequency (hereinafter, also referred to as a “bit signal”) is transformed into a transformed signal is performed, bit signals are filtered by a BPF (Band-pass filter). As a result, the bit frequencies on the negative side are filtered, and bit signals of positive-side frequencies in the UP zone and the DOWN zone are targets for the FFT process. Then, a peak signal of the positive-side frequency in the UP zone and a peak signal of the positive-side frequency in the DOWN zone are paired. In other words, originally, the peak signal of the negative side in the UP zone, which corresponds to a microscopic object, is filtered and is not paired with a peak signal of the positive side in the DOWN zone. As a result, false object data (hereinafter, referred to as “ghost data”) having a distance and a relative speed different from the original distance and relative speed of the microscopic object is derived.
More specifically, for example, when the vehicle travels at 60 km, in a case where a microscopic object is present at an extremely short distance (for example, 0.6 m on the front side of the vehicle) from the vehicle, and the microscopic object is a microscopic object such as snow or rain having a relative speed of −60 km/h corresponding to the speed of the vehicle, in the radar device, ghost data that is present at a short distance (for example, 6.64 m on the front side of the vehicle) from the vehicle and has a relative speed (for example, of −5.94 km/h (−1.65 m/s)) is detected.
As a result, there is a case where the radar device outputs ghost data that does not originally need to be output to the vehicle control device, and the vehicle control device controls the behavior of the vehicle based on the ghost data.